When an image is formed by using an electrophotographic or electrostatic imaging method, electrostatically charged images formed on a photoreceptor are developed by a toner that is electrically charged in advance through friction, and then the images are fixed. Examples of image-fixing methods are a hot-roll method for fixing the developed toner image by compressing a hot roll; and a non-contact fixing method for fixing the toner image by using an electric oven or flash beams. To go through those procedures without encountering problems, it is necessary for the toner to maintain a stable charge level and to have excellent fixability onto the paper. In addition, since an image-forming apparatus includes a heating device as the fixing component and thus its internal temperature increases, it is necessary to prevent toner blocking. Moreover, no smudges on the apparatus and no overlapping results on the printed surface should appear during continuous printing operations; namely, toner durability is required.
Furthermore, in a hot-roll method, temperature settings of the fixing component have been getting lower from the viewpoint of saving energy. Accordingly, toners are required to have properties for fixing images onto paper at lower temperatures; namely, low-temperature fixability is in great demand. In addition, apparatuses are becoming more compact, and rollers are not coated with a release agent. Thus, demand for the toner that can be released easily from the hot roll, namely, the hot offset resistance of a toner, is growing. Also, to obtain high quality images, toners are required to have smaller particle sizes and improved gloss.
Toner binder resins have a significant impact on the aforementioned toner properties. Examples of binder resins are polystyrene resins, styrene-acrylic resins, polyester resins, epoxy resins, polyamide resins and the like. Among them, polyester resins have drawn attention because of their toughness, excellent low-temperature fixability, and well balanced properties.
Preferred polyester resins for toners are those capable of achieving both fixability in a low-temperature range and hot offset resistance in a high-temperature range. To obtain such properties, widely employed methods for designing polyester resins are branching or crosslinking resins by adding a trivalent or higher carboxylic-acid or alcohol component.
To manufacture polyester resins, methods have been studied for preventing coloration of resins by using a metal-containing compound as the polymerization catalyst.
For example, Patent Literature 1 describes a polyester resin formed with a trivalent or higher carboxylic-acid or alcohol component, dicarboxylic acid, aromatic diol and aliphatic diol, as well as a method for manufacturing such a polyester resin by controlling the vacuum degree.
Patent Literature 2 describes a method for reducing the metal content to prevent coloration of polyester resin.